1. Field of the Invention
This invention relates to wire mesh support elements especially for high temperature environments, and methods for making the same.
2. The State of the Art
Devices made from knitted wire mesh are commonly used as seals, bushings, seats, and supports in exhaust systems for internal combustion engines. Such devices are used in connecting exhaust system conduits, supporting the periphery of the catalytic converter in its housing, and supporting the axial ends of the catalytic converter in its housing, among other functions.
In these types of devices, a wire is knitted into a desired mesh, and the mesh is compacted in a die into a desired geometry. The compaction is usually partial so that a structure having some porosity is produced, and the partially-compacted porous structure can be infiltrated with a high temperature resistant material, such as described in U.S. Pat. No. 5,385,873 (the disclosure of which is incorporated herein by reference). Other times the compaction results in a denser article that can be used as a bushing at the end of a catalytic converter, such as described in U.S. Pat. Nos. 4,683,010 and 6,286,840 (the disclosures of which are incorporated herein by reference). Still other devices are even more densely compacted and can be used as a filter element in an air bag and/or an exhaust assembly, as described in U.S. Pat. No. 6,277,166 (the disclosure of which is incorporated herein by reference).
In the area of bushings, seals, and supports used in combination with catalytic converters, there are two basic uses for such devices, whether or not made of wire mesh.
One type of support device spans the perimeter of the catalytic converter substrate or support, which is usually round or oval in shape, and this device supports the substrate in its metal housing, giving the housing its characteristic round or oval shape when seen from the underside of the car, although the substrate and its housing can be in any geometry. A conventional substrate is a ceramic monolith. In this environment, the device must cushion the monolith from bumps and jolts in a radial direction (with respect to the direction of the gas flow through the ceramic monolith) and provide protection from exhaust gases leaking around the monolith.
The other type of support device is used at the ends of the substrate, where the exhaust gases enter the pores of the monolith for catalytic conversion, and where the catalytic reaction products exit. In this environment, the support device must cushion the monolith from bumps and jolts in the axial direction (again with respect to the direction of the gas flow) and should direct the hot inflowing gas stream away from the perimeter of the monolith to avoid damaging the perimeter support device and bypassing the conversion process. These end location support devices can be thought of as also providing a sealing or baffling function because they deflect the hot exhaust gases from impinging on the perimeter support device and seal the gas conduit so the exhaust gases enter the catalytic converter as intended. The perimeter cushioning device may be an intumescent mat. The hot exhaust gas can erode the edge of the mat, thereby compromising its cushioning ability and eventually causing the mat to fail. Some prior art end-located support devices were comprised of a compacted element with round wire on the outside and flat wire on the inside.
Problems with wire mesh support devices used in exhaust systems are typically thermal expansion effects and corrosion effects, especially in the environment of the catalytic converter. The cold working (drawing, molding) of wire can cause hardening of the wire, which thereby affects the compression characteristics of the wire mesh element. The thermal expansion of hardened wire that does not soften upon heating can crack the ceramic monolith. On the other hand, wire that softens upon heat requires accounting for different compression characteristics at different temperatures. Yet other problems involve corrosion: wire that maintains its compression characteristics is typically not as corrosion resistant as wire that softens upon heat, which is typically more corrosion resistant.